For what it's worth I had 2x blood lactate tests in an outpatients myopathy clinic.
For the first I was told I had the highest result the doc had seen outside of ICU, 7.4 with normal range 0.6 - 2.0. Other refs I found suggested it was on par with what you'd see post-marathon. The repeat test a few months later was high-normal.
I was experiencing PEM on the first day. Before the test I'd walked 5-10mins from the train station and went up several flights of steps to the point of burning muscles and needing to use my arms to get up them (a rapid onset of that was always obvious on a "bad day"). That trip was the only thing I'd done all day, certainly no 'exercise', and I wasn't sick (no more than usual).
The second trip I was well rested, without PEM, though the train journey and walk was always still enough to do me in the next day, it just didn't impart the immediate issues.
I did get a muscle biopsy off the back of that first result. Unfortunately I was never that satisfied with the follow-up, it took months of chasing to get a report saying "There is a possibility of mitochondrial changes but the staining needs to be repeated due to some technical issues". After more months of chasing the lab apparently confirmed there were only mild, non-specific changes after all, but I never saw any written report and it sounds like someone messed something up, so who knows if it did ever get properly analysed. Unfortunately the negative biopsy meant no genetic screening & I was discharged, so dead end.